This invention relates in general to an echo suppressor circuit, and in particular, it relates to an echo suppressor circuit having application in a time assigned speech interpolation system.
A time assigned speech interpolation or TASI system is a high speed switching and transmission system which uses the idle time (i.e., silence) in telephone calls to interpolate the speech of two talkers on a single channel. One such system is disclosed in copending application Ser. No. 863,902, filed Dec. 23, 1977 and assigned to the assignee of the present invention. In TASI type systems such as the one disclosed therein, and in any system in which relatively long propagation delays are inherent, echos may be a problem. Further, when TASI type systems operate in a tandem arrangement, echo problems may be severe. An echo, as is well known in the art, is caused by the return of a talker's voice signal on a return path to that talker after some noticeable delay. To eliminate echo problems, the prior art has provided a wide variety of echo suppression circuits. These echo suppressors may be defined as highly sophisticated switches, including associated comparators and control circuitry which determine which party is talking and which then insert a high loss in the return path of that party.
So long as only one talker is active in a particular conversation at a particular time, the expedient of inserting high loss in the speaker's return path as a means of echo suppression is effective. However, a problem exists if both parties to a conversation attempt to speak simultaneously, a condition hereinafter referred to as "double talk." Since one party's return path is the other party's transmit path, the provision of a high loss in the return path of one party prevents the other party's speech signal from being properly transmitted, i.e., the other party's attempt to overtalk will be frustrated.
The prior art, as evidenced by the article entitled "ECHO SUPPRESSION DESIGN IN TELEPHONE COMMUNICATIONS" by P. T. Brady and G. K. Helder, The Bell System Technical Journal, November 1963, page 2893, at page 2913, has attempted to solve the problem of doubletalk by the provision of a control circuit which detects a doubletalk condition, i.e., the existence of speech in both directions. Then, this prior art echo suppressor provides a predetermined loss in the distant speaker's return path. The loss provided in the return path is a function of the amplitude of the distant speaker's signal. The disclosed system thus provides some attenuation for echoes in the doubletalk situation, while still providing both parties a chance to speak. However, this prior art solution does not take into account the relative volumes of the speech of both of the parties to a conversation. Thus, suppose the speaker at a distant location speaks at a high volume and a speaker at a near location speaks at a low volume. If both parties seek to speak at the same time, the predetermined loss inserted on the return path of the distant party in order to effectively suppress his echo, since it is proportional to the volume of his speech, may be so great as to mask the near speaker's attempt to overtalk. Moreover, even if this predetermined loss is not sufficient to mask the near speaker's attempt to overtalk, it two or more systems having such an echo suppressor circuit are operated in tandem, the cumulative effect of the loss on the return path of the distant speaker could very well mask the near talker's speech.
In the aforementioned TASI system, it is likely that two TASI systems could be linked in tandem and thus it is likely that a speaker having a weak voice would not be able to overtalk if known prior art echo suppressor circuits were used in each of the TASI links.
It is an object of the present invention to provide an echo suppressor circuit which minimizes the problem of doubletalk.
It is a further object of the present invention to provide an echo suppressor circuit which provides echo suppression and which allows each speaker to communicate when and if he desires. It is a still further object of the present invention to provide an echo suppressor circuit which minimizes the problem of doubletalk without obscuring the speech of a talker having a weak voice.
It is still a further object of the present invention to provide an echo suppressor circuit which provides effective echo suppression in time assigned speech interpolation systems which are operated in tandem.